In shallow water of 100 feet or less, marine structures such as offshore jackets are typically salvaged by a derrick barge which lifts and places the jacket upon a towing vessel. At greater depths, however, the offshore jacket must be refloated in either a vertical or a horizontal position before it can be salvaged. After refloating, the cumbersome structure is then either towed in the water or positioned on a barge and towed to some other location.
One method of refloating the jacket is to attach bouyancy tanks or bags to it. Unfortunately, this is a costly procedure since deploying and securing this equipment entails extensive offshore salvage and diving time.
Fortunately, most jackets can be refloated without requiring supplemental bouyancy equipment provided the jacket legs can be sealed off and deballasted. Generally, when the jacket was initially installed, the legs were equipped with top and bottom closure plates that sealed the legs for floatation purposes. To sink the structure, the closure plates were removed. Now that the jacket is to be refloated, the legs must be resealed and deballasted. Installing the top closure plate is a relatively easy matter, but the same cannot be said for the bottom closure plate or for deballasting the leg once the bottom of the leg is plugged.
There currently exist several concepts for sealing the lower region of a jacket leg. One concept employs an inflatable air bladder that fits within the jacket leg, but care must be taken not to rip or tear it, nor does the bladder provide a means for easily deballasting the leg.
A second concept, illustrated in U.S. Pat. Nos. 3,751,932, 4,215,951, 4,262,702, and 4,549,580, employs a generally cylindrical insert having an outer circumferential ring of a rubber-like material. The insert is lowered to the desired elevation afterwhich the rubber ring is biased against the inside surface of the leg. Such an insert is quite intricate (i.e. costly) and should the sealing force ever be reduced or come undone, the seal would be broken. Such an insert is also not very well suited for sealing against uneven surfaces.
Another concept is illustrated in U.S. Pat. Nos. 3,908,682, 4,262,702, and 4,518,015. This concept is similar to the preceding one except that it employs annular inflatable bladders to seal against the inside surface of the cylinder. Such bladders are inherently better at sealing against uneven surfaces but care must be taken to avoid ripping or puncturing them. Also, unless the air pressure within these bladders remains relatively constant, the changing hydrostatic pressure can rupture them. Thus the need for an air supply umbilical cord secured to the apparatus, even after refloating, entailing much additional equipment and increasing the overall cost of the plug.
It is thus an object of this invention to provide a sealing plug that is reliable, adaptable to various jacket leg diameters, able to seal against uneven surfaces, and positionable anywhere along the length of the cylinder to be plugged. It is also an object of this invention to provide a sealing plug that can be installed from the top of the jacket leg with a minimum of peripheral equipment and with no diving operation required. A further object is to provide a plug that can resist hydrostatic pressure from one side while the structure is being refloated. Another object is to provide a means for deballasting the jacket leg and for relieving any pressure build-up within the leg.